This invention relates to a single phase, two pole A.C. split-phase unidirectional induction motor of the resistance split-phase type.
Single phase induction motors have customarily been designed and utilized for fractional horsepower application and have generally been classified in accordance with the methods of starting. Several types of single phase induction motors include split-phase motors, capacitor start-induction run motors, permanent split capacitor motors, two value capacitor motors and shaded pole motors, for example.
The capacitor type motors generally employ a capacitor in series with the starting winding to provide a phase displacement of currents in the starting and running windings to produce starting torques. Such capacitors in some applications are permanently connected to the winding construction while other motors selectively remove such starting capacitors by either a speed responsive switch or a current responsive relay. Some single phase induction motors which have employed capacitors within their starting field winding have employed non-quadrature windings in which the starting field winding has been displaced out of quadrature with the main field winding in a direction opposite to the direction of rotor rotation.
Many prior resistance split-phase induction motors have employed starting windings wound to have a higher resistance than the main windings and frequently provide additional resistance connected in series therewith so that the difference in resistance of the respective main and starting winding circuits provides the requisite phase displacement for providing starting torque to the motor. Certain types of resistance split-phase constructions have employed centrifugal switches for sensing the rotor speed to disconnect the starting winding in response to the rotor reaching a predetermined speed. Such disconnection of the starting winding by a centrifugal switch ensures that the heating of the starting winding does not become excessive by the motor operating to higher speeds.
Other resistance split-phase motors and particularly those which are hermetically sealed in a compressor hve employed a current relay which responds to the current flowing through the main winding to selectively disconnect the starting winding from the energizing source in response to a sensed predetermined main winding current. With all of the coils of the starting winding wound in the forward or same direction and spaced in quadrature from the main winding coils, in such a resistance split-phase motor, the main winding current, necessary to actuate the sensing relay for disconnecting the starting winding, generally requires the motor to attain an excessive speed and consequent low torque thereby resulting in an excessive heat buildup within the starting windings. The resistance of such a starting winding could be increased in an attempt to reduce the heating thereof by adding additional forward turns to the starting winding. Such added forward turns, however, increases the reactance thereof which increases the main winding current with respect to the rotor speed so that the motor must accelerate to an excessive speed before the main winding current will decrease to a magnitude whereat the current relay will disconnect the starting winding. Thus, added forward turns only to a quadrature type starting winding increases the heating problems in the starting winding. Such an excessive heat buildup may, in fact, burn out or short the starting winding thereby rendering the motor inoperative and, in any event, the starting winding would be disconnected while the motor is providing a substantially lower torque output.
The heating problems of the generally smaller diameter wire in the starting windings used in resistance split-phase induction motors, having quadrature windings, have been substantially reduced by employing a number of reverse or back turns within the starting winding which have increased the resistance thereof to reduce the heating rate while also decreasing the mutual reactance with the main winding to ensure that the current relay disconnects the starting winding at the proper speed and torque level. Additional forward turns have to be employed with the reverse turns in the starting winding of such motors and frequently equal the number of reverse turns to maintain the desirable torque output of the motor while maintaining the heating rate of the starting winding within tolerable limits by disconnecting the starting winding at the proper speed and torque level.
While the employment of reverse or back turns and correspondingly added forward turns to the starting winding in quadrature type resistance split-phase induction motors has provided a desirable commerical motor, such structure requires the employment of large amounts of additional wire together with additional time consuming steps of fabrication, thus increasing the cost of each motor.